This study starts from setting up the finite element simulation model of B-pillar assembly. Then, the size, free size and laminate orientation of carbon fiber composite B-pillar reinforced plate were optimized based on the principles of constant stiffness. Maximum strength and displacement of the assembly were obtained through Finite Element (FE) simulation of quasi-static three-point bending test on B-pillar assembly. B-pillar sample was fabricated by a Vacuum Infusion Process (VIP) and conducted on a three-point bending test to check the strength index of the B-pillar assembly. Finally, according to 2018 C-NCAP standards, side impact performance of vehicle was elaborated. The comparison results among stiffness, flexural property, side impact intrusion, intrusion velocity and acceleration indicate that the optimization design of CFRP can replace the original B-pillar steel reinforced plate under the premise of ensuring rigidity, strength and side impact performance. CFRP B-pillar reinforcement plate is 1.376kg lighter than that of the conventional one, and the weight loss ratio is up to 76.4%.